@Marquis Harris I'm from the south shore as well (Dartmouth/NB area to be exact). Best piece of advice I can give here is don't over think it and be straight up with him. A simple question like "Would you be willing to finance all or some of the deal if I were to make an offer?". And see what his response is.
I'm big on reduced waste -- I wouldn't go crazy on researching the property, comps, etc, if you're only play is seller financing. If he answers yes or some sort of variation of it, then move to the next step and see what number makes sense for you.
Before that question though, I think there is value in just understanding the basics of seller financing 101. You want to come across as intelligent and having their best interest. Throw some basic numbers out in terms of why it's beneficial to seller finance (added interest, better to their bottom line)
I just bought a 5 family and only brought 8k to the table as the seller financed 65k. Those opportunities are everywhere, just got to ask the right questions.
Kyle